Something I like to do is to turn my -lee facedown spreadeagle and go for the ass with feathers or electric toothbrush which sometimes can give you amazing results. Also the hogtied position is an all time classic, as it is the perfect access for tickling the soles and lick the toes at the same time. Finally have you tried to roll her in a blanket like a mummy with the feet exposed? I hope those help...!